---------
-- ## base64工具类
--
-- @module Base64
-- @author Alex Kloss
-- @see http://lua-users.org/wiki/BaseSixtyFour
-- @license LGPL2

local Base64 = {}

-- character table string
local b = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

--- encoding a string to base64 format
-- @string data
-- @return encoded string
function Base64.encode(data)
    return ((data:gsub('.', function(x)
        local r, b = '', x:byte()
        for i = 8, 1, -1 do r = r .. (b % 2 ^ i - b % 2 ^ (i - 1) > 0 and '1' or '0') end
        return r;
    end) .. '0000'):gsub('%d%d%d?%d?%d?%d?', function(x)
        if (#x < 6) then return '' end
        local c = 0
        for i = 1, 6 do c = c + (x:sub(i, i) == '1' and 2 ^ (6 - i) or 0) end
        return b:sub(c + 1, c + 1)
    end) .. ({ '', '==', '=' })[#data % 3 + 1])
end

--- decoding a string from base64 format
-- @string data
-- @return decoded string
function Base64.decode(data)
    data = string.gsub(data, '[^' .. b .. '=]', '')
    return (data:gsub('.', function(x)
        if (x == '=') then return '' end
        local r, f = '', (b:find(x) - 1)
        for i = 6, 1, -1 do r = r .. (f % 2 ^ i - f % 2 ^ (i - 1) > 0 and '1' or '0') end
        return r;
    end):gsub('%d%d%d?%d?%d?%d?%d?%d?', function(x)
        if (#x ~= 8) then return '' end
        local c = 0
        for i = 1, 8 do c = c + (x:sub(i, i) == '1' and 2 ^ (8 - i) or 0) end
        return string.char(c)
    end))
end

return Base64
